test: Add python test test_qr_securejoin_broadcast, and fix some small bugs I found on the way

src/receive_imf.rs

@@ -3571,6 +3571,9 @@ async fn apply_in_broadcast_changes(
                 info!(context, "No-op broadcast 'Member added' message (TRASH)");
                 msg = "".to_string();
             } else {
+                chat.id
+                    .add_encrypted_msg(context, mime_parser.timestamp_sent)
+                    .await?;
                 msg = stock_str::msg_add_member_local(context, ContactId::SELF, from_id).await;
             }